An organic light emitting diode (“OLED”) display typically includes, in sequence: (1) a transparent anode (e.g., the anode can be comprised of indium tin oxide (“ITO”)); (2) a hole transporting layer (“HTL”); (3) an electron transporting and light emitting layer (“emissive layer”); and (4) a cathode. When a forward bias is applied, holes are injected from the anode into the HTL, and the electrons are injected from the cathode into the emissive layer. Both carriers are then transported towards the opposite electrode and allowed to recombine with each other in the device, the location of which is called the recombination zone; the recombinations in the emissive layer produce visible light. In an OLED display, each intersection of the anode and cathode is referred to as a pixel (e.g., an electroluminescent (“EL”) element) from which light can be emitted.
Anode-cathode shorts within a pixel (“pixel shorts”) are one of the major causes for failure of passive matrix OLED devices. Under multiplexed operation a pixel short becomes visible as a black pixel combined with a bright column or bright row effect. The pixel short lowers the display quality and may even render the display unusable. The pixel short can be caused by, for example, substrate imperfections or asperities, anode layer irregularities, non-uniformity of the one or more organic layers, and airborne particles introduced in the element structure during handling. In most cases the short develops suddenly after a certain operation time. Pixels with risk of shorting usually have an increased reverse leakage current at zero operation hours. It is difficult to detect this leakage current due to the noise added by the multitude of pixels that are in parallel to the pixel to be measured.
In case of OLED displays, the measurement of pixel related leakage becomes particularly important, since there is a high probability that leaky pixels will develop into pixel shorts. Similar measurements are important for other matrix-structured devices such as, for example, memory circuits, active matrix displays, detector arrays, and solar cell arrays. The leakage current of a single pixel can be very small (sometimes less than 1 nA). It is difficult and time consuming to detect such a small leakage current in each pixel of a display that has thousands of pixels. There is a need to develop a fast and accurate technique to measure pixel leakage currents less than 10 nA (or 10 uA/cm2).
Therefore, it is desirable to have a fast technique to accurately measure a signal which is a suitable indicator for the pixel leakage currents of a newly fabricated display so as to identify those pixels that are later likely to develop pixel shorts.